mmc: meson-mx-sdio: Cleanup IS_ERR() checks

Using PTR_ERR_OR_ZERO() instead of IS_ERR() works, but it's not how
you're supposed to write these conditions.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>